I have a class that takes a generic class Collection: <T: Model> (Model is a class) and a protocol (Resource) that some of the subclasses of Collection implement:
class Collection: <T: Model> {
typealias Callback = (result: Collection <T>) -> ()
}
protocol Resource {...}
Is it possible to write a protocol extension where Self is an instance of Collection?
Trying to extend the protocol with the class that takes a generic:
extension Resource where Self: Collection {
func fetch() {}
}
Gives:
Reference to generic type 'Collection' requires arguments in <...>
Trying to extend the class that takes a generic with the protocol:
extension Collection where Self: Resource {
func fetch(callback: Callback?) {}
}
Gives:
'Self' is only available in a protocol or as the result of method in a class
I'm not sure how to proceed. The goal is for the function to only be available on instances of Collection that conform to Resource.
The problem is Collection is a generic class so every where you declare it, you must attached the specialized type, like Collection<T>. However extension to a protocol can't be specified with a generic type so you end up not being able to supply T to Collection.
In your case though, T is constrained to be of type Model so why not use that in the default protocol implementation:
extension Resource where Self: Collection<Model> {
func fetch() {
// default implementation
}
}
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